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Abstract

In this work, the transport properties of metal/3C-SiC interfaces were monitored employing
a nanoscale characterization approach in combination with conventional electrical
measurements. In particular, using conductive atomic force microscopy allowed demonstrating
that the stacking fault is the most pervasive, electrically active extended defect
at 3C-SiC(111) surfaces, and it can be electrically passivated by an ultraviolet irradiation
treatment. For the Au/3C-SiC Schottky interface, a contact area dependence of the
Schottky barrier height (ΦB) was found even after this passivation, indicating that there are still some electrically
active defects at the interface. Improved electrical properties were observed in the
case of the Pt/3C-SiC system. In this case, annealing at 500°C resulted in a reduction
of the leakage current and an increase of the Schottky barrier height (from 0.77 to
1.12 eV). A structural analysis of the reaction zone carried out by transmission electron
microscopy [TEM] and X-ray diffraction showed that the improved electrical properties
can be attributed to a consumption of the surface layer of SiC due to silicide (Pt2Si) formation. The degradation of Schottky characteristics at higher temperatures
(up to 900°C) could be ascribed to the out-diffusion and aggregation of carbon into
clusters, observed by TEM analysis.

Introduction

With respect to the hexagonal silicon carbide polytype (4H-SiC), cubic silicon carbide
(3C-SiC) has potential advantages for power device applications, specifically in terms
of higher electron mobility in metal oxide semiconductor field-effect transistor [MOSFET]
channels and due to the absence of bipolar degradation upon electrical stress [1,2]. However, the low stacking fault [SF] formation energy for this polytype [3] leads to high densities of these defects, which prevent the achievement of the predicted
electrical properties. While 3C-SiC MOSFETs with excellent on-state characteristics
have been demonstrated [4], a significant reduction of the SF density is required in order to achieve acceptable
blocking voltage values and off-state leakage currents. Hence, it is crucial to better
understand which role these defects play in the non-ideal contact properties and explore
ways to mitigate their detrimental influence on devices. In this sense, imaging the
evolution of the structural and electrical properties at SiC interfaces at a nanoscale
level can be the way to better understand the physical transport phenomena and to
finally improve the performance of potential devices.

In this work, the transport properties of metal/3C-SiC interfaces were characterized
employing nanoscale techniques in combination with the conventional electrical measurements.
In particular, the nanoscale electrical activity of SFs in the semiconductor at the
contact interface was studied by conductive atomic force microscopy [C-AFM]. The effects
of a surface passivation on the localized leakage currents through SFs and the characteristics
of fabricated Au/-3C-SiC diodes were discussed. Also, the electrical and structural
properties of the Pt/3C-SiC system were studied, where high-temperature annealing
can induce metal/SiC interface reactions and strongly affect the barrier homogeneity
and leakage currents caused by semiconductor surface states.

Experimental

3C-SiC(111) heteroepitaxial 12-μm n-type ([N] = 2 × 1017 cm−3) layers were grown onto (0001) 4H-SiC substrates using a chlorine-based chemical
vapor deposition [CVD] process [5] (sample A). Homoepitaxial 4-μm n-type ([N] = 3 × 1015 cm−3) 3C-SiC layers were grown on free-standing 3C-SiC(001) substrates [6] using CVD (sample B). Ohmic back contacts were formed by evaporation of a 100-nm-thick
layer of Ni and a subsequent rapid annealing step at 950°C for 60 s to form nickel
silicide [7,8]. The 3C-SiC surfaces were then subjected to a UV irradiation treatment at 200°C in
order to electrically passivate carbon-related defect states at the surface [9,10]. Front contacts were formed by sputter deposition of a 100-nm-thick Au (samples A
and B) or Pt film (sample B), and circular contacts with radii from 5 to 150 μm were
defined by a lift-off process. The fabricated Pt diodes were annealed for 5 min at
500°C, 700°C, and 900°C in argon ambient. The structural evolution of the Pt/3C-SiC
system upon thermal annealing was studied by X-ray diffraction [XRD] and transmission
electron microscopy [TEM], which was also used to study structural defects in the
3C-SiC epilayers. XRD patterns were measured using a Bruker-AXS D5005 θ-θ diffractometer. TEM analyses were carried out after mechanical thinning and a final
ion milling at low energy (5 keV Ar ions). Bright field images were recorded at 2-20
kV by a JEOL 2010F microscope equipped with the Gatan imaging filter. The effects
of the structural evolution on the electrical behavior of fabricated Schottky diodes
were investigated by current-voltage (I-V) characterization in a Karl Süss electrical probe station, as well as by C-AFM [11], performed using a Veeco DI dimension 3100, equipped with the nanoscope V electronics
and the scanning spreading resistance [SSRM] module [12]. The parameters describing the metal/3C-SiC interfaces were extracted from I-V measurements using thermionic emission theory [13].

Results and discussion

A key factor for the conduction properties through a contact is the quality of the
metal/semiconductor interface, e.g., structural defects reaching the semiconductor
surface can lead to a localized barrier lowering and increased leakage currents [14,15]. It has recently been shown by SSRM mapping, performed in cross-section, that extended
defects in 3C-SiC cause a localized lowering of the resistance through the layer [16]. As a consequence, the crystalline quality of the 3C-SiC epilayer is pivotal for
good device operation.

The micrographs in Figure 1a, obtained by applying a bias voltage of −2 V to the C-AFM tip that is scanned in
contact mode on the semiconductor surface, show the morphology and the corresponding
current map of the as-grown 3C-SiC(111) surface. As can be seen, leakage current preferentially
flows through SFs, and several current maps determined on different areas on the sample
alongside plan-view TEM analysis (not shown here, see, e.g., [9]) showed these to be the most pervasive extended defects affecting the electrical
properties at the 3C-SiC surface and, hence, at the contact interface. In contrast,
similar current maps measured after a UV surface treatment (as described in "Experimental")
showed no localized leakage current through the SFs above the detection limit (I < 50 fA). Consequently, the electrical conduction through SFs at the 3C-SiC(111) surface
can be suppressed by UV irradiation, during which ozone is generated. UV ozone treatment
is known to remove surface defects in SiC related to carbon atoms due to oxidation
[10], and SFs arriving at the 3C-SiC(111) have a C termination [17]. Indeed, the AFM morphology map in Figure 1b, obtained on the UV-irradiated 3C-SiC surface after selective wet oxide etching,
reveals trenches of a few nanometers at the SF locations. Hence, the passivation of
the SFs may result from a preferential oxidation occurring locally inside these defects
where the polarity is shifted with respect to the Si-terminated (111) surface [17].

Figure 1.Passivation of localized leakage currents, passing through SFs at the as-grown 3C-SiC
surface, by UV irradiation. C-AFM morphology (top) and current maps (bottom) of an as-grown 3C-SiC(111) surface at a tip bias of −2 V (a) showing localized leakage currents passing through stacking faults. The AFM morphology
of the UV-irradiated 3C-SiC surface after a wet oxide etch revealed trenches in the
SFs, suggesting that their passivation is due to a local oxidation at these defects.
The I-V characteristics measured on Au/3C-SiC diodes with a contact radius of 20 μm exhibited
greatly reduced leakage currents after passivation (c).

The effect of the passivation on the I-V behavior of fabricated Au/3C-SiC(111) diodes is shown in Figure 1c. A strong reduction of the leakage current is observed, while the forward current
is largely unaffected. However, even after this passivation, a contact area dependence
of the Schottky barrier height was observed for the Au/3C-SiC system where ΦB gradually increased from 0.7 to 1.4 eV as the contact radius was reduced from 150
to 5 μm [18].

The contact area dependence of ΦB observed for the Au/3C-SiC(111) system may result from an inhomogeneous interface
between the metal and the semiconductor, or it could be caused by leakage currents
related to surface states. Independent of its origin, this dependence should be mitigated
if a 'fresh' metal/SiC interface is created instead of forming the contact interface
at the original sample surface which was exposed to chemicals, sputter damage, and
to air.

Pt is known to react with SiC at high temperatures, thus consuming a thin surface
layer of SiC to form platinum silicide, in turn forming a new interface with the SiC.
Both Pt and its silicides have high work functions that should result in good barrier
heights on 3C-SiC [19]. Hence, we investigated the properties of the Pt/3C-SiC system upon high-temperature
annealing. Previous studies on Pt contacts to 3C-SiC have reported the onset of platinum
silicide phase formation to occur at annealing temperatures ranging from 650°C to
above 750°C [20,21]. Moreover, both increased [22] and reduced [21,23] leakage currents have been reported upon silicide phase formation. Clearly, the effects
of high-temperature annealing on the structural and electrical properties of Pt/3C-SiC
is ambiguous. In this study, XRD analysis (not reported here) showed that the Pt2Si phase was formed already after annealing the Pt/3C-SiC(001) system at 500°C. The
Pt2Si phase is thermodynamically stable in the studied temperature range [22], and the XRD patterns remain essentially the same also after annealing at 700°C and
900°C. Ternary compounds are not stable, meaning that carbon must be freed during
the reaction. Indeed, the XRD spectra showed an increased presence of crystalline
carbon with increasing annealing temperature.

While XRD coupled with TEM analysis (see Figure 2) showed that all the Pt have been converted into the stable Pt2Si phase already at 500°C, higher temperature annealing gives rise to increased localized
high leakage current areas at the contact interface. Figure 3a shows the morphology of the SiC surface where the large vertical lines are due to
several stacking faults bunching together during the growth of the 3C-SiC substrate.
Comparing Figure 3a with the current map of an adjacent Pt contact (Figure 3b) determined in the same sample orientation, it is clear that the localized leakage
spots occur preferentially along the direction of stacking faults. The total area
that is covered by these leaky spots was determined from current maps measured after
each annealing temperature and increases from 12% at 500°C to 28% and 55% after annealing
at 700°C and 900°C, respectively. These leakage spots suggest a Schottky barrier inhomogeneity,
characterized by local low-barrier patches of about 0.5-1.5 μm in diameter. Clearly,
the evolution of these low-barrier patches will affect the properties of the fabricated
diodes. Indeed, the existence of low-barrier patches contributing to an overall lowering
of the average barrier is a common way of modeling non-ideal macroscale diode behavior
[24].

Figure 2.TEM images of the Pt(Pt2Si)/SiC interface. Bright-field, cross-section TEM images of the Pt(Pt2Si)/3C-SiC interface for the as-deposited Pt (a) and after annealing at 500°C (b), 700°C (c), and 900°C (d).

Figure 3.Morphology of the SiC surface and current map of an adjacent Pt contact. AFM morphology of the 3C-SiC(001) surface (a) and C-AFM current map determined at a tip bias of −5 V on an adjacent Pt contact
after annealing at 500°C (b).

Figure 4 shows localized I-V spectroscopy measured by C-AFM at 25 different tip locations (separated by 1 μm) on
the Pt2Si contacts after annealing at 500°C (a), 700°C (b), and 900°C (c). Increased local
variations are observed under both forward (barrier height) and reverse (leakage currents)
bias as the annealing temperature increases, consistent with the increasing presence
of localized low-barrier patches at the contact interface. I-V characteristics were also measured in an electrical probe after each annealing step
on circular diodes with radii of 20 and 100 μm, and the extracted diode parameters
are summarized in Figure 4d. Already, the as-deposited Pt/3C-SiC(001) contacts exhibited improved electrical
properties with respect to the Au/3C-SiC(001) system; the leakage current density
(at −3 V) measured for Au and Pt diodes fabricated on the same wafer (sample B) reduced
from 1 × 10−6 to 3 × 10−8 A/mm2. Moreover, the contact area dependence observed for the Au/3C-SiC system is absent
for the Pt/SiC interface, suggesting better interface homogeneity. As can be seen
in Figure 4d, the leakage current is slightly improved after annealing at 500°C compared to the
as-deposited Pt, whereas a strong improvement of the Schottky barrier height (from
0.77 to 1.12 eV) is observed for this annealing temperature. At higher temperatures
(700°C and 900°C), both the reverse and forward I-V characteristics begin to degrade.

Figure 4.I-V spectroscopy, measured by C-AFM, and corresponding diode parameters. Localized forward (top) and reverse (bottom) I-V spectroscopy measured by C-AFM at 25 different tip locations on the Pt2Si contacts after annealing at 500°C (a), 700°C (b), and 900°C (c). The Schottky barrier heights (black, left axis) and leakage current densities taken at −3 V (red, right axis) extracted from I-V probe measurements for the different annealing temperatures are shown in (d).

To understand the electrical evolution upon annealing, the cross-section of the contact
interface was studied by TEM. As seen in the TEM images in Figure 2 the Pt layer (Figure 2a) has reacted completely after the 500°C anneal (Figure 2b), and the consumption of Pt results in the formation of a polycrystalline Pt2Si layer, characterized by the presence of interfacial protrusions penetrating into
the underlying SiC [22]. However, additional changes are observed at the higher temperatures. At 700°C (Figure
2c), larger size variations are visible in the Pt2Si protrusions and there is a formation of a carbon layer at the interface and carbon
clusters inside the protrusions (also confirmed by energy-filtered TEM). After annealing
at 900°C, this layer is thicker and more pronounced carbon clusters are observable
inside the protrusions.

The TEM observations can be consistently correlated to the previously shown electrical
results. Since Pt2Si is the only phase observed by XRD for the different annealing temperatures, the
evolution of the electrical properties after annealing above 500°C is likely unrelated
to the silicide phase formation. More likely, the changes in the electrical properties
can be related to changes in the density of states at the contact interface. As reported
by Mullins and Brunnschweiler [25], for the as-deposited contacts, the surface states are presumed to behave as donor-like
traps, generated during the sputtering of the metal. Hence, after annealing at 500°C,
the interface moves away from the original 3C-SiC surface and these states no longer
affect the properties of the contact, causing a widening of the energy barrier that
in turn causes the tunneling leakage current to reduce. The gradual degradation observed
at higher annealing temperatures can be explained by the increased amount of carbon
clusters at the contact interface and in the Pt2Si protrusions due to incomplete diffusion of carbon upon further consumption of SiC,
which has been shown to become an issue at annealing above 600°C [23]. The agglomeration of electrically active carbon clusters at the interface gives
rise to barrier inhomogeneities, characterized by local low-barrier patches, ultimately
leading to an increase of the leakage current [26].

Conclusion

Nanoscale electrical and structural characterization of metal/3C-SiC interfaces were
performed using C-AFM, XRD, and TEM. It was shown that the most pervasive extended
defect at 3C-SiC(111) surfaces, the stacking fault, can be passivated by UV irradiation
treatment. This allowed an almost ideal Schottky behavior to be measured for the Au/3C-SiC
system when characterizing very small diodes. However, a contact area dependence of
the Schottky barrier height was found after this passivation, indicating that other
electrically active defects and/or inhomogeneities at the contact interface still
remain even after UV passivation. The contact area dependence of ΦB was absent for the Pt/3C-SiC system, which also showed improved electrical properties
with respect to the Au/3C-SiC system. Annealing of Pt/3C-SiC at 500°C resulted in
further reduction of the leakage current and an increase of the Schottky barrier height.
These changes are attributed to a consumption of the surface layer of SiC due to Pt2Si formation. However, upon annealing at higher temperatures (700°C and 900°C), a
degradation of the Schottky characteristics occurred. TEM analysis showed that this
degradation can be ascribed to the aggregation of carbon clusters at the interface.

Competing interests

The authors declare that they have no competing interests.

Authors' contributions

JE designed the experiments, carried out the sample preparation, performed the electrical
measurements and drafted the manuscript. FG and PF supported JE during scanning probe
microscopy measurements; RLN performed the XRD measurements and analysis; SR and SL
provided the 3C-SiC samples. FR and VR coordinated the research activity and helped
design the experiments. All authors took part in the discussion of the results and
helped shape the final manuscript. All authors read and approved the final manuscript

Acknowledgements

This work has been supported by the European Commission in the framework of the MANSiC
project (MRTN-CT-2006-035735). The authors are grateful to C. Bongiorno for his assistance
during TEM sample preparation and analysis and to S. Di Franco for his help with the
fabrication of Schottky diodes.

References

Lebedev AA: "Heterojunctions and superlattices based on silicon carbide".